1. Field of the Invention
The present invention relates to a lead zirconate titanate-based piezoelectric film, a piezoelectric element using the piezoelectric film, and a liquid discharge apparatus.
2. Description of the Related Art
An actuator including an ink jet recording head is provided with a piezoelectric body having a piezoelectric property that expands and contracts with variation in applied electric field intensity, and a piezoelectric element provided with an electrode that applies an electric field to the piezoelectric body.
In recent years, in order to meet the demand for miniaturization of an apparatus, miniaturization of an actuator in cooperation with a semiconductor process technology such as a microelectromechanical systems (MEMS) technology has proceeded. In the semiconductor process technology, high-precision processing using film formation or photolithography becomes possible. Therefore, there has been actively conducted research on thinning of a piezoelectric body in an actuator.
As a piezoelectric material having high piezoelectric properties, a lead zirconate titanate (PZT)-based perovskite oxide has been widely used due to its performance. It is known that when a PZT-based perovskite oxide piezoelectric film has a morphotropic phase boundary (MPB) composition in which Zr:Ti is near 52:48, the piezoelectric constant and the electromechanical coupling coefficient thereof become maximum, which is appropriate for actuator applications.
In JP2012-99636A, it is described that in a piezoelectric element provided with a piezoelectric thin film having a laminate of a lead titanate layer and a lead zirconate layer, which have columnar structures, the compositions of lead titanate and lead zirconate in the piezoelectric thin film are caused to be MPB compositions, thereby improving piezoelectric properties.
On the other hand, as a technique for improving piezoelectric properties using a method other than that for the formation of the MPB compositions, a PZT-based piezoelectric film is doped with various donor ions having higher valences than those of substituted ions. Since the ionic valence of Zr and Ti in B-site is 4, as donor ions that substitute for B-site elements, B-site elements having an ionic valence of 5 or higher, such as V, Nb, Ta, Sb, Mo, and W have been used.
However, when attempting to increase the amount of B-site elements to be substituted, the crystallization temperature of the perovskite oxide is increased, and thus it is difficult to increase the doping amount of the donor ions. In J. Am. Ceram. Soc, 84 (2001) 902 and Phys. Rev. Let, 83 (1999) 1347, it is described that in a case of Nb, the doping amount for B-site is about 2.4 mol % to 20 mol %, and when a large amount of Nb is doped, the crystallization temperature rises to 800° C. or higher.
In JP3791614B, it is described that in order to dope a PZT-based ferroelectric film with Nb as B-site ions at a high concentration, 0.5 mol % or more of Si is added. Si is a sintering aid for obtaining a thermal equilibrium state by accelerating sintering in a thermal equilibrium process by a sol-gel method, and is necessary for suppressing an increase in crystallization temperature due to the Nb doping. However, when the sintering aid is added, piezoelectric properties deteriorate, and it is thought that the effect of the addition of donor ions cannot be sufficiently exhibited. Furthermore, as the amount of Si increases, cracks are likely to occur due to the relationship with stress. Therefore, only a thin film having a film thickness of 1 μm or less can be formed, and it is difficult to achieve high properties.
On the other hand, regarding the formation of a donor ion-doped perovskite oxide film according to a vapor phase growth method, a sintered body target, of which the sintered density is increased by adding Si as a sintering aid to the target in an amount of from 0.1 mol % to less than 5 mol % during the production of the target for the purpose of solving a problem of the generation of particles during vapor phase film formation due to a reduction in the sintered density of the target used for film formation according to the vapor phase growth method depending on the kind of doping donor ions or acceptor ions and a problem of breaking of a large target which is likely to occur during film formation, is disclosed in JP2003-63860A.
However, in JP2003-63860A, evaluation of only leakage current characteristics due to the addition of Mn is performed using a Mn-doped strontium titanate sintered body target produced by adding Si as the sintering aid in an amount of from 0.1 mol % to less than 5 mol %, and there is neither description nor suggestion regarding the effect on a PZT-based perovskite oxide film, and furthermore, the effect of improving piezoelectric properties. Moreover, since the sintering aid is contained in the thin film obtained by using the sintered body target of JP2003-63860A in an amount of from 0.1 mol % to less than 5 mol %, it is thought that the piezoelectric properties are deteriorated due to the presence of the sintering aid and the effect of the addition of donor ions cannot be sufficiently exhibited.
An attempt to dope PZT with Nb at a high concentration without the use of a sintering aid has been reported by the inventors. In JP5367242B, a Nb-doped PZT film in which the effect of the addition of donor ions is significantly exhibited by controlling film formation conditions in a non-thermal-equilibrium process is described. In JP5367242B, the production of the Nb-doped PZT film having a MPB composition succeeded.